ABSTRACT. Many biominerals contain micro-or nano-crystalline mineral components, organized accurately into architectures that confer the material with improved mechanical performance at the macroscopic scale. We present here a new effect, which enables to observe the relative orientation of individual crystals at the sub-micron scale. We call it polarization-dependent imaging contrast (PIC), as it is an imaging development of the well-known x-ray linear dichroism. Most importantly PIC is obtained in situ, in pristine biominerals. We present here PIC in the prismatic and nacreous layers of Haliotis rufescens (red abalone), confirm it in geologic calcite and aragonite, and corroborate the experimental data with theoretical simulated spectra. PIC reveals new and unexpected aspects of nacre architecture that have inspired theoretical models for nacre formation.2 INTRODUCTION. Nacre, or mother-of-pearl, is intensely studied by materials scientists, mineralogists, physicists as well as chemists, because of its remarkable mechanical properties and its fascinating and poorly understood formation (1,2). Nacre is a composite of layered 400-nm thick aragonite tablets (3), and 30-nm thick organic matrix layers (4,5). Aragonite, an orthorhombic CaCO 3 polymorph, is hard but brittle. Aragonite accounts for 95% of nacre's mass, leading one to expect the mechanical characteristics of nacre to be similar to those of aragonite, yet nacre is 3000 times more resistant to fracture than aragonite (6). Materials scientists have only recently began to learn how to prepare synthetic composites outperforming their components by such large factors, and do so inspired by nacre (7,8,9,10), although not as efficiently and orderly organized as natural nacre. It is therefore of extreme interest to understand and possibly harness the mechanisms of nacre formation. Here we report unprecedented observations on the structure and architecture of nacre enabled by the use of x-ray absorption near edge (XANES) spectroscopy (11), combined with PhotoElectron Emission spectroMicroscopy (X-PEEM)(12). These observations inform and inspire new theoretical models for nacre formation mechanisms (13).

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Angle-dependent x-ray absorption near-edge structure (XANES) and scanning photoelectron microscopy measurements were performed to differentiate local electronic structures at the tips and sidewalls of highly aligned ZnO nanorods. The overall intensity of the O K-edge XANES spectra is greatly enhanced for small photon incident angles. In contrast, the overall intensity of the Zn K-edge XANES is much less sensitive to the photon incident angle. Both valence-band photoemission and O K-edge XANES spectra show substantial enhancement of O 2p derived states near the valence band maximum and conduction band minimum, respectively. The spatially resolved Zn 3d core level spectra from tip and sidewall regions show the lack of chemical shift. All the results consistently suggest that the tip surfaces of the highly aligned ZnO nanorods are terminated by O ions and the nanorods are oriented in the [0001̄] direction.

O K -, ZnL3, and K-edges x-ray absorption near-edge structure (XANES) spectra and scanning photoelectron microscopy (SPEM) spectra were obtained for ZnO nanorods with various diameters. The analysis of the XANES spectra revealed increased numbers of O2p and Zn4p unoccupied states with the downsizing of the nanorods, which reflects the enhancement of surface states when the diameter is decreased. Valence-band photoemission spectra show a significant narrowing of the valence band for the 45nm diameter nanorod. The Zn3d intensities in the Zn3d SPEM spectra are drastically diminished for all nanorods as compared to the ZnO reference film, which can be interpreted as a reduction in density of itinerant final states or in transition probability.

Metrics & MoreArticle Recommendations * sı Supporting Information ABSTRACT: "Spin" has been recently reported as an important degree of electronic freedom to improve the performance of electrocatalysts and photocatalysts. This work demonstrates the manipulations of spin-polarized electrons in CsPbBr 3 halide perovskite nanoplates (NPLs) to boost the photocatalytic CO 2 reduction reaction (CO 2 RR) efficiencies by doping manganese cations (Mn 2+ ) and applying an external magnetic field. Mn-doped CsPbBr 3 (Mn-CsPbBr 3 ) NPLs exhibit an outstanding photocatalytic CO 2 RR compared to pristine CsPbBr 3 NPLs due to creating spinpolarized electrons after Mn doping. Notably, the photocatalytic CO 2 RR of Mn-CsPbBr 3 NPLs is significantly enhanced by applying an external magnetic field. Mn-CsPbBr 3 NPLs exhibit 5.7 times improved performance of photocatalytic CO 2 RR under a magnetic field of 300 mT with a permanent magnet compared to pristine CsPbBr 3 NPLs. The corresponding mechanism is systematically investigated by magnetic circular dichroism spectroscopy, ultrafast transient absorption spectroscopy, and density functional theory simulation. The origin of enhanced photocatalytic CO 2 RR efficiencies of Mn-CsPbBr 3 NPLs is due to the increased number of spin-polarized photoexcited carriers by synergistic doping of the magnetic elements and applying a magnetic field, resulting in prolonged carrier lifetime and suppressed charge recombination. Our result shows that manipulating spin-polarized electrons in photocatalytic semiconductors provides an effective strategy to boost photocatalytic CO 2 RR efficiencies.